An amplifier, a constant-current circuit and the like each having cascode-connected transistors can provide high output impedance, and thus can reduce the property fluctuation caused by power supply voltage variation. In addition, the use of a semiconductor integrated circuit having cascode-connected transistors as an amplifier can decrease a feedback from an output side to an input side, and thus can improve the high frequency properties. For these reasons, the amplifier and the constant-current circuit having the cascode-connected transistors are frequently used in a variety of fields. Japanese Patent Application Publication No. 2001-156558 discloses a cascoded current-mirror circuit.
Recently, the amplifiers and the constant-current circuits operating at a low voltage have been demanded with the progress of the miniaturization, high integration, and low power consumption of the semiconductor integrated circuits. However, in a conventional cascoded current-mirror circuit and a conventional cascoded amplifier, transistors are cascode-connected between a higher voltage source and a lower voltage source, thereby causing a problem of having a small power supply voltage margin. Additionally, in the cascoded current-mirror circuit, when cascoded-connected transistors each having a multi-finger structure are formed in separate active regions, this configuration causes a problem of enlarging the area of a pattern layout of the amplifier or the constant-current circuit.